QComboBox::view() will give a QAbstractItemView pointer. Since the view is a list view, you can cast it safely to QListView. Use QListView::setSpacing() to something of your choice. Default value is 0.
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QAbstractItemView *abVw = comboInst->view();
QListView listVw = qobject_cast<QListView>(abVw);
if (listVw) {
listVw->setSpacing(2);
}
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